1. Technical Field
The present invention relates to a configuration and an operating method of an optical communication system in which plural subscriber devices share an optical transmission line, and a system extension such as an extension of a transmission distance or an increase in the number of subscribers in the system.
2. Related Art
A demand for communication using broadband has been increased, and an access line for users has increasingly undergone transition from an access technology based on a telephone line such as digital subscriber lines (DSL) to a large-capacity access line using optical fibers. At present, in the access line service, passive optical network (PON) systems have been frequently used from the viewpoints of the line laying costs and maintenance costs. For example, the standardization technology in the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) is recited as a typical example (ITU-T Recommendation G.984.3). Since about 2006, introduction of a gigabit PON (GPON) into the access network has been started in the respective countries.
In the PON, an optical signal is split or multiplexed by the aid of an optical fiber and an optical splitter, and transmitted and received between an optical line terminal (hereinafter referred to as “OLT”) and an optical network unit (hereinafter referred to as “ONU”). A communicatable distance between the OLT and the ONU has a given limit distance due to attenuation of the optical signal that passes through the optical fiber and a light intensity attenuation caused by optical split through the optical splitter. As a specific example, in the case of the GPON, generally, a communication interval is set to 20 km at a maximum, and the number of splits (the number of ONUs connectable to the OLT) is set to 64.
With an increase in chances where standard home users access to an internet through an access circuit to collect information, and chances where the users use public service through a communication line, an enlargement of an access network provision area (service scale) is demanded for each carrier. That is, while an increase in the number of access line users is facilitated, measures (business investment) for increasing the number of users for each subscriber station are required to accommodate new users. As a method of increasing the number of users for each station, there are proposed a method in which a user accommodating device (PON) is added (that is, an OLT is added), or a method in which the number of users for each PON system, that is, the number of ONUs is expanded.
The PON is configured so that all of complicated system control for preventing the optical signals from being mixed together, and ONU management are implemented by the OLT. For that reason, in general, the OLT is far more expensive than the ONU. Further, the costs for newly laying the optical fiber result in huge carrier expense. In view of the above circumstances, a desirable solution is to increase the number of ONUs for each OLT.
On the other hand, for enlargement of the transmission capacity in the access network, transmission at a higher bit rate than that in the related art has been studied. In the ITU-T and the IEEE (Institute of Electrical and Electronics Engineers), the standardization of the respective next-generation PONs which are called “10 gigabit PON” (10GPON) and “10 gigabit Ethernet (registered trademark) PON” (10GEPON) has been advanced. In the high bit rate transmission of this type, the attenuation of the optical signal that passes through the optical fiber and the wavelength dispersion become larger than those in the transmission at the related-art bit rate. Accordingly, in order to construct a system having the same communication distance as that of the existing PON, the system needs to have a light receiving device having a wide dynamic range, a high performance (low attenuation and low dispersion) optical fiber, and a dispersion compensating function, resulting in a remarkable increase in the costs. The number of users can be increased due to the higher bit rate whereas an increase in the development costs is problematic.
JP-A-2009-065443 discloses a communication system that synchronizes a time of a terminal device with a time of a transmission device, and a device thereof.